30 July 2014

Sun and Moon and Ebb Tides

A kayaking friend of mine referred me to a question posed by a fellow kayaker regarding tides in and around the Golden Gate: Why are Ebb Tides strongest in SF Bay at night during the
summer and during the day in winter? I love this kind of question, where the celestial mechanics of the Solar System impact the daily experiences of hobbyists and average people on Earth.

First of all, why are there differences in size of high & low tides throughout the month and year? Each month the highest highs and lowest
lows occur when the Moon is Full or New. That’s pretty simple. But there are
two important factors regarding the Moon’s proximity to Earth on any given Full
or New Moon. And there is one important factor regarding the Sun’s proximity to
Earth on any given Full or New moon.

1. The Sun’s Changing Proximity

The Earth’s orbit around the Sun is elliptical, meaning that
there is a close approach each year on January 4th (“perihelion”) and
a corresponding far point in our orbit in July (“aphelion”). The New and Full
Moon phases just before or after January 4th have higher high tides
at the noontime tide, leading to a very fine ebb tide in the middle of the
afternoon.

2. The Moon’s Changing Proximity

The Moon’s orbit around the Earth is elliptical, so there
are times when the Moon is closer (“perigee”) and farther (“apogee”). Lately
the press has made much to do about the “Super Moons” that occur when the Full
Moon lines up with Perigee. On August 10, 2014 we will have a very fine Super
Moon precisely at the same time as Perigee, so there will be higher high tides
at midnight and lower low tides at dawn, leading to a very fine ebb tide in the
middle of the night.

Another factor is the Moon’s location along its orbit from a
north-and-south perspective. The Moon’s orbit around the Earth is inclined
slightly and follows the “ecliptic” which also meanders above and below the
plane of the equator, meaning that sometimes it is physically above the plane
of Earth’s equator and sometimes below (“ascending and descending nodes” of the
lunar orbit). If the Moon is above the plane of the equator, it is actually a
bit closer to the land and water north of the equator, exerting a slightly
greater tidal pull. The next peak of this effect is on August 9, 2014, so we
can look forward to some amazing high and low tides at the next Super Moon. The
ascending node and its impact are not tied directly to winter or summer, so
this effect is less predictable.

- - - - - - - - - -

To answer the original question: there are good ebb tides
each month at New and Full Moon, but the changing effects of perihelion and
perigee cause the solar influence to be greater in Winter (hence the better
daytime ebb tides) and the lunar influence to be greater in Summer (hence the
better nighttime ebb tides).

About Me

In 2005 I began writing a column for the San Francisco Waldorf School newsletter called "The Urban Astronomer." I started this blog in 2007 as a place to archive my articles and to offer additional insights on the night sky - even if you live in a big city. In 2008 I became an occasional guest on the KFOG Morning Show, and more recently on KALW and KGO. Archived shows are posted on the blog.